Test set for communication apparatus



April 16, 1946. w. KONRAD TEST SET FOR COMMUNICATION APPARATUS Filed Feb. 29, 1944 IN VEN TOR H15 ATTORNEY Patented Apr. 16, 1946 TEST SET FOR COMDIUNICATION APPARATUS William L. Konrad, Pi

The Union Switch vale, Pa.,

ttsburgh, Pa., assignor to & Signal Company, Swissa corporation of Pennslyvania Application February 29, 1944, Serial No. 524,427 2 Claims. (Cl.179--175.1)

My invention relates to test sets for communication apparatus, and more particularly to test sets for railway train communication apparatus.

There are railway train communication systems which use carrier telephone and signaling currents. The transmitting circuit includes the track rails and communication between'a station on a train and stations remote therefrom is accomplished through circuit elements electrically cou pled to the track rails. In this way, telephone communication is carried on between a wayside station and a train, or between two trains, or between two vehicles of the same train, such as, a locomotive and acaboose of a freight train.

Testing of such communication apparatus is required if the system is tobe maintained at its maximum eifectiveness. Obviously, equipment installed one, locomotive, a caboose or other railway car can-be most conveniently tested at the terminals, classification yards and roundhouse. In all such places poor track conditions and high noise energy may prevail and to test the equipment on one vehicle with'the equipment on a second' vehicle would require a specially prepared stretch of track. Such an arrangement would necessitate that all equipped locomotives, cabooses and other cars be switched to this special prepared track, and this would result in extra work and delay. Furthermore, a second equipped vehicle may not be available just when needed.

Because of these circumstances, a feature of my invention is the provision of novel and improved test sets for communication apparatus.

Another feature of my invention is the provision of novel portable test sets for railway train communication apparatus, the set being portable, it can be taken to the immediate vicinity of the equipment to be tested, and a satisfactory electrical relationship between the test set and the tested equipment can be obtained.

Again, a feature of my invention is the provision of a test set for communication apparatus incorporating novel means to power the set from either of two sources, such as 60 cycle 110 volt alternating current, or 32 volt direct current. Also, the set incorporates novel means wherewith it operates at either of the two carrier frequencies used for train communication systems of the type here involved and wherewith the output of its transmitter can be varied and indicated for checking the sensitivity of the receiver of the equipment being tested.

Other features, objects and advantages of my invention will appear as the specification Drogresses.

The features, objects and advantages embody-' ing my invention I accomplish by providing in portable form a receiver and a transmitter'of carrier telephone current, and a dual power unit, together with switching means for establishing different operating conditions of the set.

The receiver and transmitter are of the electron tube type and to minimize the apparatus an oscillator and a final stage amplifier are made common to the receiver and transmitter.

Terminals are positioned on the test set for connections to a portable pick-up coil, 3, portable sending circuit, a telephone hand set, and to either an alternatingcurrent or a direct current power ource; r

With such an arrangement the test set can be taken to the vicinity of a vehicle having mounted thereon equipment to be tested and the pick-up coil and thesending circuit disposed for effective relationship with the vehicle apparatus. lithe equipment to be tested is on a locomotive at a roundhouse or on acarin a repair or cleaning yard where commercial cycle alternating current is usually available, the power unit on the test set would be connected to it. g If the car is out in the yard away from alternatingcurrent lines, then a 32 volt battery or line would be used to power the test set, such battery and direct current line being usually available about railway yards since it is standard for train lighting. To condition the test set to receive orto send, a multiple pole two position switching device is incorporated in the set, and the contacts of this device are interposed in the plate circuits of the receiver and transmitter tubes, the control grid circuit for the final stage tube, and the connection of the output of the final stage amplifierto the hand set terminals and to the sending circuit terminals, a first position of this switching device conditioning the apparatus to receive and a second position conditioning the apparatusfor sendmg. 7 i 7 Since train communication systems of the type here involved may use a carrier of either 5700 cycles or 7700 cycles, different'capacitors and inductors are incorporated in the set to adjust the receiver, transmitter and oscillator to operate at either'5700'or 7700 cycles. Such capacitors and inductors are connected into the circuits through different contact members of a second multiple pole two position switching device, one position of which device conditioning the set to operate at 5700 cycles and its'second position conditioning the set to operate at 7700 cycles.

Furthermore, such systems usually' provide 'a in testing the sensitivity of th receiver of a train.

carried equipment.

The power unit of the test set is constructed rent as a source of supply and it incorporates a special power transformer, a tuned alternator r A secondary winding and a switching device. v of the power transformer is connected through a rectifier and a conventional filter to the plate circuits of the several electron tubes for. effective.

One primary energization of such plate circuits. windingv of this power ,transformeris connected to-the alternating current supply terminalsof the set and is proportioned to supply proper plate voltage for the tubes in response to a 60 cycle 110 volt alternating current supply. A second primary winding of the power transformer is connected to contacts of the tuned alternator, the operating winding of which is connected to the direct current supply terminals ofzthe test set. Theparts are proportioned to supply proper plate. voltage in response to operation of the tuned: alternator from a 32 volt source o'fdirect current. Contact members of a' multiple pole two position switching device are interposed in the connections of these primary windings and tuned alternator and arranged so; that current from: only one pair of. supply terminals: can be received at a time; Also on ,andoff switches are preferably included in; these connections. The filaments of the-several tubes of setarearranged in a circuit for the tubesto be properly heated' from either the alternating; current. orthe direct current. source; ,A contact member or the last'mentioned, switching device is: included in the filament circuit to connect the filament'to the direct currentterminals when direct current power isused and to connect the filament circuit to atleast a portion of the second primary winding of, the power transformer when alternating; current supply is used,.the second primarywinding thusfunctioning as a;-secondary winding to provide proper heating currentfor the filaments.

An adjustable indicating means is included-sin the connection to the sending circuit terminals of the set todetermine and indicate the: mag-. nitude of. the output and thereby checkfthe sen.- sitiivity of the receiver of the equipment under tes a I shall. describe one form of apparatus embodying my invention,,and shall then point out the novel features thereof in claims. I

The accompanying drawing is a diagrammatic view showing one form of apparatus fora test set embodying my invention when used for railway train communication apparatus;

It is to be understood that my invention is not limited in its use to railway train fcommunication apparatus and this one application serves to illustrate the many places where apparatus embodying my inventioniis useful.

Referring. to' the drawing, atest set :isindicated as a whole by the dashan'd. dotline TS.

This test set comprises an assembly of apparatus which is mounted on' a chassis or panelsuitably the test to use either alternating current or direct cur:

housed in a case provided with handles so that the set can be carried from one place to another. The specific form of case is immaterial and it is not shown.

Terminals I to l2, inclusive, are located on the panel of the test set for connection to outside circuit elements which comprise a pick-up coil PC, a handset including a telephone transmitter or microphone MC and a telephone receiver TR, .a sending circuit SC and a power extension cable indicated by the broken lines WI and W2.

The apparatus of the test set TS comprises a receiver, a transmitter, an oscillator, a final stage amplifier, an indicating device and a power unit together with several switching devices as its essential elements.

The receiver includes a first stage amplifier tube V'l': and associated circuits, a demodulator tube V2 and associated circuits, and a filter Fl.

The first stage amplifier tube VI may be of any one of several types of tubes and is shown as an indirectly heated pentode, its control grid l9 andcathode 20 being connected to secondary winding 18: of an input transformer Tl through a biasing unit BUI. Primary winding. l5 of transformer TI and a condenser I! in multiple are connected through a condenser 1-6 to terminals II and 12, to which terminals pick-up coil PC is arranged to be connected. A condenser 2! is connected in multiple with condenser I6 through a contact member BI of a multiple pole two position switching device indicated conventionally by a dotted line BS, such switching device being described more fully hereinafter. Also, a condenser 22 is connected'in multiple with condenser ll through contact member B2 of switching device BS. The parts are proportioned so that when switching device-BS isplaced at its first position, that is, in the position where the contact members BI and B2 occupy the position illustrated by the solid lines in the drawing, and thecondensers 2 land 22 areincluded in the circuit, the first stage amplifier is conditioned toreceive a frequency band extending from 6200 to. 8000; cycles per second, such frequency band being that-of, an upper side band telephonecur rent of'a carrier of 5700 cycles modulated by a Voice band. of 500 to 2300 cycles. When switch BS is moved: to its second position to disconnect condensers 2i and 22, the first stage amplifier is conditioned to receive a frequency band of 8200 to 10,000. cycles per second, such band being the upper side band telephone current of a carrier of 7700' cycles modulated by voice frequencies extendingfrom 500-to 2300 cycles.

Tube. Vi is provided with a plate circuit including wire 24 connected to a power unit to be described hereinafter, a resistor 25, plate 23 and intervening tube space to cathode 20 of tube Vi and biasing unit BUI to ground electrode 20 connected to the other terminal of the power unit. This plate circuit is coupled to the demodulator tube V2 through a transformer T2, a primary winding 2i of which. transformer is coupled to the plate circuit of tube V! through a condenser 28. and a secondary winding 29 of which transformer. is connected to the control elements 30 and SI of tube V2, tube V2 being shown as a twin triode type of tube- A mid terminal of winding 200i transformer T2 is connected by wire 32 to' a carrier oscillator to be described hereinafter and the cathodes 33 and 34 of tube V2 are connected in multiple to ground electrode 35. Plate voltage for'plates 36 and 31 of tube V2 is supplied from the power unit through wires 24 and.38 and 4 56 and wire 51,

the two portions of primary winding 39 of a transformer T3, the connection being that of the usual push-pull arrangement. Windings 39 and 40 of the transformer T3, together with condensers 4| and 42 and a resistor 43 form a low pass filter Fl proportioned to reject the side band frequencies and to pass the voice frequency band here assumed for illustration as extending from- 500 to 2300 cycles. The output side of filter Fl is connected to control grid 45 of a final stage amplifier tube V3 through wire 44 and contact member D| of'a multiple pole two position switching device indicated conventionally by a dotted line DS and which device will be explained more fully hereinafter.

Amplifier tube V3 is provided with a plate circuit including wire 46 connected to the power unit as will appear shortly, winding 41 of an output transformer T4, plate 48 and intervening tube space to cathode 49 of tube V3, biasing unit BU2 and ground electrode 50, which is connected to one terminal of the power unit. A secondary winding of transformer T4 is connected to terminals 3 and 4 through wires 52 and 53, a contact member D2 of the switching device DS being interposed in wire 53, terminals 3 and 4 being those provided for connection of the telephone receiver TR of the handset.

It follows that with switching devices BS and DS set at their respective first positions, that the positions shown in the drawing, a carrier telephone current of 6200 to 8000 cycles per second picked up by the coil PC is amplified at tube VI, demodulated at tube V2, voice frequencies passed at filter Fl, the voice frequencies amplified at tube V3 and the speech reproduced at the telephone receiver TR. When switching device BS is set at its second position and the switching device DS at its first position, then a carrier telephone current of 8200 to 10,000 cycles will be received, demodulated and amplified and the speech reproduced at the telephone receiver TR.

The transmitter of the test set includes an input transformer T5, a modulator tube V4, a filter transformer T6 and final stage amplifier tube V3. Winding 18 of transformer T5 is connected across terminals I and 2 of the test set through wire 1'! and ground electrodes 19 and 54, a, contact member D3 of the switching device DS being interposed in this circuit. Microphone MC of the hand set is adapted to be connected to terminals I and 2 and this microphone circuit just traced is provided with power from the power unit through a connection including resistors 55 arid as will appear shortly. Winding 58 of transformer T5 is connected to the control electrodes 59 and 60 of tube V4, tube V4 being shown as a twin triode tube. A resistor 6| is connected across winding 58 and the mid terminal of resistor 6| is connected to cathodes 64 and 65 in multiple of tube V4 through two resistors 62 and 63 in series. The junction terminal of resistors 62 and 63 is connected to a ground electrode 66 and the mid terminal of resistor 6| is connected through wire |3 to the carrier oscillator, this input circuit arrangement for tube V4 serving as a balanced circuit by which the carrier and the voice frequencies are mixed at tube V4 for modulation. Plates 6'! and 63 of tube V4 are connected to the outside terminals of winding 69 of transformer T6, a mid terminal of winding 69 being connected to'the power unit through wire 10.

Transformer T6 is included in a tuned filter which also includes condensers H and 12 and a resistor 13 connected across winding 69 and condensers 74 and 75 connected across'winding 76 of the transformer. Contact members B3 and B4 of the second switching device BS are interposed in the connections of condensers T2.and 15, respectively, and the parts are proportioned so that whenthe switch BS is placed at its first position and condensers 12 and T5 are included in the circuits, the upper side band of the carrier of 5700'cyc1es modulated by the voice frequencies is passed by the filter, and when the switch BS is placed at its second position so that the condensers l2 and proportioned to pass the upper side band of a carrier of 7700 cycles modulated by the voice frequencies.

The output of this filter transformer T6 is applied to the control grid 45 of amplifier tube V3 through wire 80 and contact member DI of the switching device DS when that switching device is placed at its second position. At least a portion of winding 5|of output transformer T4 is connected across terminals 5 and 6, to which terminals the sending circuit SC is connected. Specifically, one terminal of winding 5| is connected to terminal 5 through wire 53, contact member D2 of switch DS at its second position and wire 8|, and a preselected intermediate terminal of winding 5| is ccnnectedto terminal 6 through wire 82 and an adjustablqresistor 83.

IAn indicating meanscomprising a lamp 84 and a resistor 85 in multiple is connected across winding 5| in multiple with the sending circuit, the

. connection of this indicating means being com switching device, BS

' adjustable resistor 83 pleted at the lefthand position of a switch 86. Switch 86 whenmoved to its right-hand position disconnects the indicating means and connects winding 5| to the terminal 6 around the adjustable resistor 83.

It follows that voice frequencies created in microphone MC are mixed with the carrier in tube V4 and the upper side band passed through the filter transformer T6 and the upper side band is further amplified at tube V3 and the amplified output is applied to the sending circuit SC. When is placed at its first position to connect condensers" and 15, the transmitter is conditioned to use a carrier of 5700 cycles and when the switch BS is placed at its second position and the condensers l2 and 75 are disconnected, the transmitter is conditioned to use a carrier of 7700 cycles. It is to be noted, of course, that the transmitter is active only in the event the switching device DS is placed at its second position. When the switch 86 is placed at its left-hand position, lamp 84 by its brillianc will indicate the relative magnitude of the output current It is apparent, of course, that the lamp 84 can be replaced by an indicating meter. Also the by its adjustment varies the magnitude of the current supplied to the sending circuit and resistor dicate the magnitude of the current;

and K2. Each of these coil assemblies KI and K2 includes three windingsQthe windings of assembly Kl being designed for use when a carrier of 5700 cycles is to be supplied and the windings of assembly K2'being designed for use when a carrier of 7700 cycles is to be supplied. The windings of assembly Kl are connected to the circuits of tube V5 through contact members B5 and B6 of switch BS and a contact member CI of another switching device indicated conventionally by a 15 are disconnected, the filter is 83 can be calibrated to indottedline CS. The circuits to. be used: for oscillator tube V5 may be any oneoi several wellknown arrangements for electron tube oscillators, and it is sulficient for thisapplication to point out that plate voltage is provided through wire 89 connected to the power unit and when switch CS is placed at its first position and switch BS at its first position, the windings 90 and 9I of assembly KI areconnected into the circuits for tube V5 and a carrier of 5700 cycles is createdin the plate cirsuit of the tube. Whenswitch CS is placed at its first position for a contact memberCZ to occupy its full line position and switch BS is placed at. its second position to move contact membersB5 and B6 to their dotted line positions, then windings 93 and 94 of the assembly K2 are connected into the circuits for tube V5 and a carrier of. 7700 cycles is created. Again, when a carrier of 5700 cycles is being created andswitch CS is moved to its second position to move contact member CI to its dotted line position winding 92 of assembly KI is substituted for winding 90, and the carrier is shifted a preselected number of cycles, such as 1050'cycles, to provide a calling. current of 6750 cycles. Also whena carrier of 7700 cycles is being created and switch CS is placed at its second position to move contact member C2 to its dotted line position, winding 95 of assembly K2 is substituted for winding 93 and a calling current of 8750 cycles is created, that is, the carrier of 7700 cycles is shifted 1050 cycles.

The carrier thus created in the plate circuit of tube V5 is passed through condenser 95, resistor 9'! and wire 32 to the winding 29 of transformer T2 for use in the demodulator tube V2 of the receiver, and is passed through wire I3 to the inputcircuit for the tube V4 for use for modulation in the transmitter.

The power unit comprises a powertransformer T1, a tuned alternator TA and a full wave rectifier tube V5, together with a switching device indicated: conventionally by a dotted line AS. The outside terminals of secondary winding 98 of transformer T1 are connected to the anodes 99 and I09, respectively, of tube V6, and a mid terminal of winding 98 is connected to ground electrode I91. through resistor 55. Consequently whenan alternating. electromotive force iscreated in winding 98 and the top terminal of winding 98 as viewed-in the drawing ispositive, current flows through the left-hand portion of rectifier tube Vfito cathode I02, filter I03, wire I04, wire 46 and the plate circuit of tube V3 to ground electrode 50=-and thence through ground electrode IOI and resistor 55 to'the mid'terminal of winding 98. In multiple with this plate circuit of tube V3- isthe plate circuit for the oscillator tube V5, the circuit being completed through wire 99 and the ground electrodes I05 and IOI. Further-- more, with the switch DS- placed at its first position other paths for the rectified current are completed through the contact member D4 of switch DS and the-plate circuit of the amplifier tube VI and the plate circuit of the demodulator tube V2. When switch DS is placed at the second position to bring contact member D4 to the dotted line position, an alternative path for the rectified current is completed through the plate circuits of the modulator tube V4. During the half cycles of. the alternating electromotive force created inwinding 98 that the lower terminal of the winding is positive, then current flows through the right-hand portion of the rectifier tube V5 and the several plate circuits described above. In other words; an alternating electroascaasr motive forceinduced in winding 98 of power transformer T1 is rectified and current is supplied to the various plate circuits of the. tubes of the test set, current being always supplied to the plate circuits for tubes V3 and V5 and to the receiver tubes VI and V2 when switch DS is placed at its first position and to the plate circuits of the transmitter tube V4 when the switch DS is placed at its second position.

A first primary winding I06 of transformer T1 is connected to the alternating current supply terminals I and B through a contact member AI of switching device AS and an on and off switch I01. Consequently when the switch AS is at its first position and switch I01 is closed and an alternating current supply is connected to the terminals 1 and 8 through cable WI--W2, rectified current is applied to the plate circuits of the tubes. The parts are proportioned for the proper voltage to be appliedto the several plate circuits when the alternating current supply is one of cycles, 110 volts.

An operating winding of tuned alternator TA is connected across terminals 9 and I9 through contact member A2 of switching device AS when placed at its second position, and an "on and off switch I is closed, and the alternator is set into operation. With the alternator TA operated to alternately close contacts I09 and III], the two portions of secondary Winding II I of transformer T1 are alternately energized and an electromotive force is induced in secondary winding 93. The parts are proportioned so that when a 32 volt direct current source is connected to terminals 9 and I0, the alternator is operated properly for the electromotive force induced in winding 98 to be of proper voltage for energizing the plate circuits of the several tubes.

The filaments of the several tubes of the test sets are includedin a circuit arrangement adapted to be supplied with current from either power source. This filament circuit is shown at the lower portion of the drawing for the sake of simplicity, and each filament is given the reference character of the respective tube, plus the letter F. In case switch AS is placed at its second position for use of a direct current supply, a connection can be traced from terminal 9 through contact member A3 of switch AS, wire II2, the several filaments of the tubes to ground electrode II3 and thence to ground electrode IOI and switch. I98 to terminal I0. In case the switch AS is placed at its first position for using an alternating current supply, a connection can be traced from the lower terminal of winding II I of transformer T1 through contact member A3 of switch AS, wire II2, the several filaments, ground elec= trodes H3 and IOI and to an intermediate terminal of winding III, and winding III functions as a secondary winding for supplying proper voltage for heating the filaments.

It is to be noted that the alternating current supply can be used to power the plate and filament circuits only when switch AS is placed at its first position and direct current can be used only when the switch AS is placed at its second position.

Also, it is to be noted that the plate currents flowing through resistor 55 create a. voltage drop across this resistor, and which voltage drop is efiective to energize the microphone circuit through the connection including wire 51 and resistor 56, as explained hereinbefore. Again, it

is to be pointed out that the switching devices AS, BS, DS and CS would preferably be multipole double throw switches, each provided with a manually operable lever mounted on the panel of the test set. However, these different switch; ing devices may be a relay having aplurality of different contact members and energized by either alternating current or direct current through a push button properly identified and mounted on the panel of the test set. Furthermore, eachof the switches 86, Ill! and I 08 would be provided with a push button properly mounted on the panel for manual operation of these 1 switches. In describing the operation of the test set, I shall assume that communication equipment mounted on a locomotive is to be tested, the locomotive being at a roundhouse undergoing the usual servicing in preparation for a trip. The set TS would be placed in the vicinity of the locomotive and the pick-up coil PC placedin inductive relation to the rails of the track on which the locomotive is standing. The sending circuit SC would be connected to the rails through the medium of proper connections indicated at H6 and. H1. In place of such connection to the rails, the sending circuit can be made a loop circuit and placed in inductive relation tothe receiver of the locomotive. The cable WI-W2 would be plugged in on a commercial 60 cycle line at the roundhouse and the switch AS of the set placed at its first position, with the result that the set is properly powered from the alternating current source, the Winding II I of transformer T7 serving as a secondary winding to provide energy for the filament circuit of the tubes. The equipment on the locomotive would then be placed in an active condition. I shall further assume that the locomotive equipment is one designed for use of a carrier of 5700 cycles and thus the switch BS would be placed at its first position to condition the test set for using such carrier frequency. To test the receiver of the locomotive equipment, the switch DS would be placed at its second position to render active the transmitter of the test set. By speaking into the microphone MC, a carrier telephone current is supplied to the sending circuit which is now positioned to transfer energy inductively to the receiver of the locomotive. The magnitude of the current sent out by the test set can be adjusted through the medium of resistor 83, which is provided with a manually operable lever on the panel for adjustment thereof, and which lever preferably moves over a calibrated scale that indicates the magnitude of the outgoing current. Furthermore, by positioning switch 86 at the left-hand position, the brilliancy of the illumination of the lamp 84 will be a relative indication of the magnitude of the outgoing current. Since, as mentioned hereinbefore, the calling current has been found to be convenient for determining the sensitivity of a receiver of train carried equipment, the switch CS could be placed at its second position to shift the carrier and provide the calling current of 6750 cycles. It is to be noted that when switch CS is shifted, then the connection of modulator tube V4 is changed at a contact member C3 of switch CS for effective transmission of this calling current through the transmitter.

To test the transmitter of the locomotive equipment, the switch D8 of the test set is placed at its first position and the test set conditioned for receiving a carrier telephone current of 5700 cycles. By listening at the receiver TR, the operator can determine the relative condition of the transmitting apparatus of the locomotive.

It is to be pointed out that the amplifier tube VI of the receiver is provided with a volumecontrol including a resistor 5 provided with a suitable manually operable adjusting lever toaid in determining the relativecondition of the transmitter of the locomotive-equipment. I

In thecase the equipment .to be testedis on a caboose or car in a yard where a commercial 60 cycle current line is not available, then the cable Wl-.W 2 wouldbe connected to terminal 9 and i0 and-.tora volt storage battery or line and switch-AS moved to its second position. The operation of;;the apparatus for testing. would be carried out in amanner explainedhereinbefore. In the event the equipment to be tested is designed for operation on a carrier of 7700 cycles, then the test set would be adjusted by proper positioning of the switch BS.

Although I have herein shown and described 7 but one form of test sets for communication apparatus embodying my invention, it is under-' stood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thu described my invention, what I claim is:

1. In a test set for communication apparatus using a carrier telephone current of either a first or a second carrier frequency, the combination comprising, an electron tube amplifier-demodulator having an input circuit including capacitors adjustable to receive telephone current of either said first or second carrier frequency, an electron tube amplifier-modulator having an output circuit including capacitors adjustable to send telephone current of either said first or seccnd carrier frequency, an electron tube as cillator including inductors adjustable to upply a carrier of either said first or second carrier frequency, a first stage amplifier tube, a pair of telephone receiver terminals, a pair of sending circuit terminals, a power unit operable to supply current at a voltage as required by the plate circuit of said tubes; a first two position switching device having a first contact member connected to said capacitors of the amplifier-demodulator, a second contact member connected to the capacitors of said amplifier-modulator and a third contact member connected to the inductors of said oscillator to condition the test set to use telephone current of either said first or second carrier frequency; and a second two position switching device having a first contact member to connect a control grid of said final stage amplifier tube either to said amplifier-demodulator or to said amplifier-modulator, a second contact member to connect said power unit either to the plate circuits of said amplifier-demodulator tubes or to the plate circuit of said amplifier-modulator tubes, and a third contact member to connect the output of said final stage amplifier tube either to said telephone receiver terminals or to said sending circuit terminals to condition said test set to either receive or send.

2. In a communication apparatus test set including an electron tube transmitter and an electron tube receiver and having a first pair of terminals for being powered from an alternating current source and a second pair of terminals for being powered from a direct current source, the combination comprising, a power unit including a three winding trans-former and a tuned alternator, a first one of said transformer windings having connections to said first pair of terminals, a second one of said transformer windings having connections to 'said second pair of vterminals through contact of said alternator, athird one of said transformer winding having connections to the 'plate circuits of said transmitter and. receiver tubes, an operating winding of said alternator having connection to said second pair of terminals, a filament circuit of said transmitter and receiver tube having "connections to said second transformer winding and to said second pair of terminals; and a three pole two position switch having a first contact member interposed in said connections of said first transformer winding and closed ;at a first position of said switch, a second contact-member common to said connections of said second transformer winding and-said operating winding of the alternator and closed at a second position of the switch, and a third contact member interposed in the connections of said filament circuit to connect-that circuit-tosaidsecondtransformer winding at said first position of the switch and to connect the circuit to said second pair of terminals at said second positionof the witch. r 5 r Y WILLIAM L. KONRAD. 

